Large-scale amplification of a magnetic field by turbulent helicity fluctuations

In this paper the procedure of large-scale averaging of the magnetic-field diffusion equation with the α-term curlα(r,t)B(r,t) is used to show that a nonuniform distribution of the turbulent helicity fluctuations (more precisely, the fluctuations of the coefficient α) with a zero average value gives rise to large-scale amplification of the initial magnetic field. A detailed study is carried out of the dependence of the resulting large-scale α effect on the characteristics of the correlator 〈〈α(r, t)α(r″,t″)〉〉 in a rotating medium with a nonuniform distribution of the angular velocity ω=ω(ρ,z) (ρ is the distance for the rotation axis z). The effect of helicity fluctuations and the diffusion coefficient on the turbulent diffusion process is also investigated. Zh. éksp. Teor. Fiz. 116, 85–104 (July 1999)     

Comparison of methods for calculating turbulent diffusion coefficients

Different methods for calculating the turbulent diffusion coefficient D _T of a passive scalar impurity in an infinite homogeneous isotropic stationary turbulent medium are examined. The values of D _T calculated by these methods are compared for two limiting types of turbulence, viz., turbulence with a δ-function spectrum and turbulence with a Kolmogorov-type spectrum. The temporal dependence of the velocity correlators is assumed to be exponential. It is shown that the most accurate method is based on the use of the solution of the nonlinear equation for the averaged Green’s function with consideration of the contribution from the four-point turbulent velocity correlators. A comparison with the results of other methods that are simpler from the mathematical standpoint shows that some of them also permit the calculation of D _T with relatively good accuracy. Zh. éksp. Teor. Fiz. 111, 871–881 (March 1997)     

Thermodynamic theory of the equation of state of materials

A universal derivation of the thermodynamic equations on the basis of a combined analysis of the exact relations for any material — the virial theorem, the shock adiabat, and the differential thermodynamic identity relating the thermic and caloric equations of state of matter — is given. This combination makes it possible to reduce the fundamental problem of thermodynamics to a boundary-value problem of mathematical physics. Analytic relations T _s=T(P _s,ρ _s and T _s=T(D,u) are obtained for classical systems. Zh. Tekh. Fiz. 68, 1–9 (October 1998)     

Onset of a nonoptimal hopping regime for ac conductivity in amorphous gallium antimonide

A possible explanation has been found for the typical discrepancy between the parameters of localized states as determined from the static and dynamic hopping conductivities in tetrahedral amorphous semiconductors. It is shown for the example of a-GaSb that the Mott hopping length R _opt, the correlation length for nonoptimal hops L _T , and the ac hopping length R _ω are related as R _opt <L _T <R _ω , as a result of which the Mott law holds for the dc conductivity and the Zvyagin regime of nonoptimal hops holds for the ac conductivity σ(ω). The observed value of σ(ω) is two orders of magnitude lower than the conductivity calculated by the Austin-Mott formula for the parameters of localized states found from dc measurements. A model that quantitatively describes the static and dynamic conductivity of a-GaSb with the aid of a single set of parameters characterizing the Miller-Abrahams resistor network is proposed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 4, 322–327 (25 February 1997)     

Diffractive S and D wave vector mesons in deep inelastic scattering

Helicity amplitudes for diffractive leptoproduction of the S and D wave states of vector mesons are derived. A dramatically different spin dependence for production of the S and D wave vector mesons is predicted. It is found that R=σ_L/σ_T is very small and that the higher twist effects in the production of longitudinally polarized D wave vector mesons are abnormally large. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 268–273 (25 February 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

K*0 and φ meson production in proton–nucleus interactions at ?s=41.6\textGeV\sqrt{s}=41.6\text{GeV}

The inclusive production cross sections of the strange vector mesons K^*0, K̄^*0, and φ have been measured in interactions of 920 GeV protons with C, Ti, and W targets with the HERA-B detector at the HERA storage ring. Differential cross sections as a function of rapidity and transverse momentum have been measured in the central rapidity region and for transverse momenta up to p_T = 3.5 GeV/c. The atomic number dependence is parametrised as σ_pA=σ_pN*A^α, where σ_pN is the proton–nucleon cross section. Within the phase space accessible, α(K^*0)=0.86±0.03, α(K̄^*0)=0.87±0.03, and α(φ)=0.96±0.02. The total proton–nucleon cross sections, determined by extrapolating the differential measurements to full phase space, are σ_pN→K*0=(5.06±0.54) mb, σ_pN→K̄*0=(4.02±0.45) mb, and σ_pN→φ=(1.17±0.11) mb. For all resonances the Cronin effect is observed; compared to the measurements of Cronin et al. for K^± mesons, the measured values of α for φ mesons coincide with those of K⁺ mesons for all transverse momenta, while the enhancement for K^*0/K̄^*0 mesons is smaller.     

Lattice Dislocations in a 1-Dimensional Model

The spectral properties of the Schr?dinger operator T(t)=−d ²/dx ²+q(x,t) in L ²(ℝ) are studied, where the potential q is defined by q=p(x+t), x>0, and q=p(x), x<0; p is a 1-periodic potential and t∈ℝ is the dislocation parameter. For each t the absolutely continuous spectrum σ _ac (T(t))=σ _ac (T(0)) consists of intervals, which are separated by the gaps γ _n (T(t))=γ _n (T(0))=(α _n ⁻,α _n ⁺), n≥1. We prove: in each gap γ _n ≠?, n≥ 1 there exist two unique “states” (an eigenvalue and a resonance) λ _n ^±(t) of the dislocation operator, such that λ _n ^±(0)=α _n ^± and the point λ _n ^±(t) runs clockwise around the gap γ _n changing the energy sheet whenever it hits α _n ^±, making n/2 complete revolutions in unit time. On the first sheet λ _n ^±(t) is an eigenvalue and on the second sheet λ _n ^±(t) is a resonance. In general, these motions are not monotonic. There exists a unique state λ₀(t) in the basic gap γ₀(T(t))=γ₀(T(0))=(−∞ ,α₀ ⁺). The asymptotics of λ _n ^±(t) as n→∞ is determined. Received: 5 April 1999 / Accepted: 3 March 2000     

Trajectory attractor of a reaction-diffusion system with a series of zero diffusion coefficients

We study a reaction-diffusion system of N equations with k nonzero and N − k zero diffusion coefficients. More exactly, the first k equations of the system contain the terms a _i Δu _i − f _j (u, v), i = 1, …, k, with the diffusion coefficient a _i > 0. The right-hand sides of the other N − k equations contain only nonlinear interaction functions −h _j (u, v), j = k + 1, …, N, with zero diffusion. Here u = (u ₁, …, u _k ) and v = (υ _k+1, …, υ _N ) are unknown concentration vectors. Under appropriate assumptions on the interaction functions f(·) and h(·), we construct the trajectory attractor of this reaction-diffusion system. We also find the trajectory attractors , δ = (δ ₁, …, δ _k ), for the analogous reaction-diffusion systems having the terms δ _j Δυ _j − h _j (u, v), j = k + 1, …, N, with small diffusion coefficients δ _j ⩾ 0 in the last N − k equations. We prove that the trajectory attractors converge to (in an appropriate topology) as δ → 0+. Dedicated to the memory of Vladimir Borovikov Partially supported by the Russian Foundation for Basic Research (projects nos. 08-01-00784 and 07-01-00500).     

Correlation between the electric-field effect and weak-link type in YBa2Cu3−x Oy and YBa2Cu3−x Oy /Agx high-T c superconducting ceramics

An experimental study is reported of the effect of an electric field E⩽120 MV/m and of temperature T on the critical current I _c and I-V characteristics of yttrium-based high-T _c superconducting ceramics. The materials studied were copper-deficient ceramics, YBa₂Cu_3−x O_y (D samples), and YBa₂Cu_3−x O_y/Ag_x ceramics [S samples with silver present in amounts equal to the copper deficiency (0⩽x⩽0.4)]. It has been established that in D samples at 77 K, the electric field increases I _c and reduces substantially R for I>I _c, whereas in S samples no field effect is observed. Measurements of the I _c(T) dependence near the critical temperature showed that they can be described for all samples by a relation of the type I _c =const(1−T/T _c )^α, where α≈1 for the D samples, and α≈2 for the S samples. The results obtained suggest that the electric-field effect correlates with the existence in the ceramic of SIS-type weak links at grain boundaries. Fiz. Tverd. Tela (St. Petersburg) 40, 1195–1198 (July 1998)     

Diffusion study of nitrogen implanted into <Emphasis Type="Italic">α</Emphasis>-Hf using the nuclear resonance technique

The diffusion of nitrogen in α-Hf was studied in the temperature range of (823–1123) K using the ion implantation and nuclear resonance techniques. The measurements show that the diffusion coefficients follow the Arrhenius behavior D(T)=D ₀exp (−Q/RT) with D ₀=(5.5±2.1)×10⁻⁷ m²/s and Q=(228±1) kJ/mol. A comparison of the present results with the previous one is done.     

Influence of γ irradiation on the electrical conductivity and dielectric properties of β TlInS2 in the temperature range of the incommensurate phase

The influence of γ irradiation on the electrical conductivity σ, the dielectric permittivity ɛ, and the dielectric loss tangent tanδ of β-TlInS₂ crystals is investigated in the temperature range of the incommensurate phase. It is established that γ irradiation lowers the values of ɛ and tanδ over the entire investigated temperature range and decreases the maxima of the σ(T) and tanδ(T) curves in the vicinity of the transition temperature T _c, but does not alter the transition temperature. Fiz. Tverd. Tela (St. Petersburg) 39, 1088–1090 (May 1997)     

Implications of time-reversal symmetry for the band structure of single-wall carbon nanotubes

When electron states in carbon nanotubes are characterized by two-dimensional wave vectors with the components K ₁ and K ₂ along the nanotube circumference and cylindrical axis, respectively, then two such vectors symmetric about a M-point in the reciprocal space of graphene are shown to be related by the time-reversal operation. To each carbon nanotube there correspond five relevant M-points with the following coordinates: K ₁⁽¹⁾ = N/2R, K ₂⁽¹⁾= 0; K ₁⁽²⁾ = M/2R, K ₂⁽²⁾= −π/T; K ₁⁽³⁾= (2N −M)/2R, K ₂⁽³⁾= π/T; K ₁⁽⁴⁾= (M + N)/2R, K ₂⁽⁴⁾= -π/T, and K ₁⁽⁵⁾= (N − M)/2R, K ₂⁽⁵⁾= π/T, where M and N are the integers relating the chiral, C _h , symmetry, R, and translational, T, vectors of the nanotube by N R = C _h + M T, T = |T|, and R is the nanotube radius. The states at the edges of the one-dimensional Brillouin zone, which are symmetric about the M-points with K ₂ = ±π/T, are shown to be degenerate due to the time-reversal symmetry.     

Mechanism of giant magnetoresistance in intermetallic compounds of rare-earth ions and actinides

Giant positive or negative magnetoresistance is calculated in a band model. The spectra of the band electrons in a two-sublattice antiferromagnetic intermetallic compound depend on the antiferromagnetism vector L(T,H). The metamagnetic transition to the ferromagnetic phase is accompanied by splitting with respect to the spin σ, displacement of the energy bands, and a decrease in the effective masses of the band electrons. This mechanism of giant negative magnetoresistance is also accompanied by an increase in the relaxation time τ_jσ. Scattering by chemical-bond fluctuations is considered as the main relaxation mechanism. Giant positive magnetoresistance results from a four-subband model of 4f and 5f intermetallic compounds. The electron effective masses m _jσ(J _jT ) of the (j,σ) bands increase with the mean angular momentum J _1T (T,H) of an ion in the jth sublattice of 4(5)f ions. The thermodynamics of such a four-sublattice model, the nonlinear magnetization and magnetoresistance curves, and the nonmonotonic dependence of the specific heat C _m(T,H) on the field H are calculated. Fiz. Tverd. Tela (St. Petersburg) 39, 1806–1814 (October 1997)     

Temperature dependences of the high-field magnetization of dilute frustrated ferrimagnetic spinels

Results are presented of an investigation of the magnetic properties of dilute frustrated ferrimagnetic spinels Li_0.5Fe_2.5−x Ga_xO₄ (x=0.8–1.2), which characterize the main parameters of the ferrimagnetic state and provide evidence of local violation of collinear spin ordering and frustrations. In particular, measurements were made of the concentration dependences of the magnetic moment n ₀(x) and the Curie point T _c (x), the magnetization isotherms σ _T (H) at T=4.2 K and H⩽10 kOe, and also the low-and high-field magnetization polytherms σ _H (T). It was established that for x⩾0.8 in fields exceeding the technical saturation field H _s ∼2 kOe, the temperature dependences of the high-field magnetization σ _H (T) between 4.2 and 230 K cannot be described by the Bloch T ^3/2 law whereas this law is satisfied for undiluted Li spinel (x=0). Over the entire temperature range (4.2–230 K) the experimental curves σ _H (T) may be approximated by σ _H (T)=σ ₀(1−AT ^3/2 − BT ^5/2) for x=0.8–1.0 and σ _H (T)=σ ₀[1−CT ^3/2exp(μ(H−H ₀)/k _B T)] for x=1.1, 1.2, where μH ₀∼15 K is the internal field produced by competition between exchange interactions and frustrations. Fiz. Tverd. Tela (St. Petersburg) 40, 1075–1079 (June 1998)     

Influence of Thomson effect on the resultant local Seebeck coefficient in thermoelectric composite materials

The resultant local Seebeck coefficient α _R (=α _S−α _T) at the interface of a thermoelement has not yet been measured, although it is an important factor governing the thermoelectric efficiency, where α _S is the local Seebeck coefficient and α _T is the one caused by the Thomson effect. It is shown in this paper that α _S, α _T, and α _R of the p- and n-type Cu/Bi–Te/Cu composites are obtained analytically and experimentally on the assumption that the local temperature of the composite on which the temperature difference ΔT is imposed varies linearly with changes in position along the composite. They were indeed estimated as a function of position from the local experimental data of R,ΔI,ΔT, and V generated by applying an additional current of ±I to the composite, where R is the electrical resistance and ΔI is a current generated by the composite. As a result, it was found that the absolute values of α _S at the hot interface of the p- and n-type composites are approximately 1.5 and 1.4 times higher than their lowest values in the middle region of the composite, respectively, while those of α _T are less than 8% of α _S all over the composite and are so small that the relation α _R≈α _S can be held. We thus succeeded in measuring α _R at the interfaces of the composite.     

Elastic,micro- and macroplastic properties of polycrystalline beryllium

The Young’s modulus and the internal friction of beryllium polycrystals (size grain from 6 to 60 μm) prepared by the powder metallurgy method have been studied as functions of the amplitude and temperature in the range from 100 to 873 K. The measurements have been performed using the composite piezoelectric vibrator method for longitudinal vibrations at frequencies about 100 kHz. Based on the acoustic measurements, the data have been obtained on the elastic and inelastic (microplastic) properties as functions of vibration stress amplitudes within the limits from 0.2 to 30–60 MPa. The microplastic deformation diagram is shown to become nonlinear at the amplitudes higher than 5 MPa. The beryllium mechanical characteristics (the yield strength σ _0.2, the ultimate strength σ _u , and the conventional microscopic yield strength σ _y ) obtained with various grain sizes are compared. At room temperature, all the parameters satisfactorily obey the Hall-Petch relationship, although there is no complete similarity. The temperature dependences are quite different, namely: σ _0.2(T) and σ _u (T) decrease monotonically during heating from room temperature to higher temperatures; however, σ _y (T) behaves unusually, and it has a minimum near 400 K. The different levels of stresses and the absence of similarity indicate that the scattering of the ultrasound energy and the formation of a level of the macroscopic flow stresses in beryllium occur on dislocation motion obstacles of different origins.     

Temperature dependence of the heat capacity and saturation magnetization of dilute reentrant ferrimagnetic spinels

Using model objects consisting of dilute reentrant spinels Li_0.5Fe_2.5−x Ga_xO₄ with x=1.0–1.2, this paper describes the temperature dependence of the magnetic contribution to the heat capacity, C _m (T), at H=0 and temperatures exceeding the freezing temperature (T _f ∼10 K) and, for T⩾4.2 K, the low-field magnetization σ _H (T) and the saturation magnetization σ _S (T), as well as the magnetization isotherms σ _T (T) in fields of up to 10 kOe. The specific features of the behavior of the overall characteristics of C _m (T) and σ _S (T) are found for the states of a frustrated ferrimagnetic that occur in the conditions considered here (H=0 and T>T _f or T⩾4.2 K and strong magnetic fields) and are discussed in connection with changes in the magnetic excitation spectrum caused by local breakdowns of collinear spin ordering and frustrations. Zh. éksp. Teor. Fiz. 113, 1339–1349 (April 1998)     

Effect of γ irradiation on the low-temperature electrical conductivity and dielectric properties of TlGaSe2 crystals

Electrical conductivity and dielectric properties of single-crystal TlGaSe₂ have been studied as a function of γ irradiation dose in the 100–280 K range including the existence of an incommensurate phase. Anomalies in the form of maxima have been observed in the σ=f(T), tan δ=f(T), and ɛ=f(T) curves at the points of transition from the paraphase to incommensurate (IC) phase, T _i, and from the IC to commensurate phase, T _c. The increase in the quantities σ, tan δ, and ɛ observed initially with increasing irradiation dose is followed by their strong decrease and disappearance of the anomalies. It has been established that γ irradiation does not affect the phase transition temperatures T _i and T _c. Fiz. Tverd. Tela (St. Petersburg) 40, 1328–1331 (July 1998)     

Critical point phenomena in the electrical resistivity of binary liquid systems: CS2 + CH3CN and CS2 + CH3NO2

Electrical resistance measurements are reported on the binary liquid mixtures CS₂ + CH₃CN and CS₂ + CH₃NO₂ with special reference to the critical region. Impurity conduction seems to be the dominant mechanism for charge transport. For the liquid mixture filled at the critical composition, the resistance of the system aboveT _c follows the relationR=R _c−A(T−T _c) ^b withb=0·6±0·1. BelowT _c the conductivities of the two phases obey a relation σ₂−σ₁=B(T _c−T)^β with β=0·34±0·02, the exponent of the transport coefficient being the same as the exponent of the order parameter, an equilibrium property.     

Nonadiabatic effects in the phonon spectra of superconductors

The nonadiabatic corrections to the self-energy part Σ_s(q, ω) of the phonon Green’s function are studied for various values of the phonon vectors q resulting from electron-phonon interactions. It is shown that the long-range electron-electron Coulomb interaction has no direct influence on these effects, aside from a possible renormalization of the corresponding constants. The electronic response functions and Σ_s(q, ω) are calculated for arbitrary vectors qand energy ω in the BCS approximation. The results obtained for q=0 agree with previously obtained results. It is shown that for large wave numbers q, vertex corrections are negligible and Σ_s(q, ω) possesses a logarithmic singularity at ω=2Δ, where Δ is the superconducting gap. It is also shown that in systems with nesting, Σ_s(Q, ω) (where Q is the nesting vector) possesses a square-root singularity at ω=2Δ, i.e., exactly of the same type as at q=0. The results are used to explain the recently published experimental data on phonon anomalies, observed in nickel borocarbides in the superconducting state, at large q. It is shown, specifically, that in these systems nesting must be taken into account in order to account for the emergence of a narrow additional line in the phonon spectral function S(q, ω)≈−π ⁻¹ Im D _s (q, ω), where D _s (q, ω) is the phonon Green’s function, at temperatures T<T _c . Zh. éksp. Teor. Fiz. 115, 1799–1817 (May 1999)